AN2690

Main characteristics and circuit description

5/31

chosen to obtain a compromise between the transformer size and the harmonics of the

switching frequency, optimizing the input filter size and the total solution cost. The MOSFET

is a 800 V rated, STP10NK80ZFP, housed in the TO-220FP package, needing just a small

heat sink. The transformer is layer type, using a standard ferrite size EER35. The

transformer, designed according to the EN60950, is manufactured by TDK. The flyback

reflected voltage is ~130 V, providing enough room for the leakage inductance voltage spike

with still margin for reliability of the MOSFET. The rectifier D8 and the Transil D4 clamp the

peak of the leakage inductance voltage spike at MOSFET turn-off.

The output rectifiers are two dual center tap Schottky diodes (D7 and D5) in parallel. They

have been selected according to the maximum reverse voltage, forward voltage drop and

power dissipation. The snubber made up of R14, R66 and C8 damps the oscillation

produced by the diode capacitance and the leakage inductance. A small LC filter has been

added on the output, filtering the high frequency ripple.

D17, R75-R78, Q10 and Q15 implement an output voltage "fast discharge" circuit

discharging quickly the output capacitors when the converter is turned off. It has been

implemented to quickly decrease the residual output voltage after the converter is turned off

at no load.

Startup sequence

The circuit is designed so that at startup the flyback starts first, then it turns on the PFC

manage at startup the full output power over the entire input voltage range because it must

guarantee the regulation of the output voltage even during a load transition when the load is

increasing, but the PFC is still not yet delivering the nominal output voltage. Of course this

condition can be maintained only for short time, typically tens of milliseconds, because the

flyback is not designed to sustain this condition from a thermal point of view. The flyback

controller L6566A pin #1 (HV) is directly connected to the DC bulk voltage and at startup, an

internal high voltage current source charges C32 and C33 until the device turn-on voltage

threshold is reached, then the high voltage current source is automatically switched off. As

winding (pins 8-9) provides the voltage to power the IC. Afterwards, according to the load

level, monitored by the COMP pin, the L6566A activates the L6563, powering it via the

Because the L6566A integrated HV startup circuit is turned off, it is not dissipative during

normal operation and contributes considerably in reducing power consumption when the

power supply operates at light load. This reduction is significant in meeting worldwide

standards currently required for standby power.

Brownout protection

Brownout protection prevents the circuit from working with abnormal mains levels. It can be

easily achieved using pin #16 (AC_OK). Q3, C23, R62 and R64 implement a circuit sensing

the rectified input voltage peak value and feed it into the pin #16. An internal comparator

then enables the IC operations if the mains level is correct, within the nominal limits. If the

has been set at the voltage reached by the bulk capacitor after the hold-up time. The internal

comparator has in fact a hysteresis allowing to set independently the L6566A turn-on and

turn-off voltage. Sensing the input voltage before the input rectifier allows faster restart

because there is no need to wait for the bulk capacitor discharge.